It is known that logs are paper rolls produced by winding machines, by means of which a paper web is wound around a tubular cardboard core. Each log is then divided into various rolls having inferior lengths and corresponding to the standard commercial formats. The division takes place with a succession of cuttings carried out along a plane which is orthogonal to the longitudinal axis of the logs. Machines called “cutting-off machines” are used for this purpose.
A cutting-off machine for paper logs usually features a structure with a horizontal platform, which is provided with a plurality of feeding channels for the logs to be cut off, means for moving the logs along the respective feeding channels, and cutting means cut the logs as said before. Cyclically, each log is positioned adjacent to the cutting means, then it is submitted to cutting, and finally is moved forward to carry out the subsequent cutting.
JP-I0058382 describes a cutting-off machine for paper logs in which the cutting means consist of a double bevel annular blade positioned horizontally and vertically moved during the cutting of the logs. More precisely, the blade is band-shaped and features a bevel both on its upper and lower sides, and it is wound on two vertical-axis pulleys, so that each bevel defines a horizontal ring. The logs are positioned on a conveyor provided with two overlapped planes which are oriented orthogonally to the blade. The pulleys on which the blade is wound are supported by a structure which is connected to respective lowering and lifting means. The logs are cyclically disposed in the cutting position, the blade is lowered, then the logs are pushed forward again and the blade is lifted. The logs are cut during the lowering and lifting movements of the blade. FIGS. 1A and 1B show two cutting steps: FIG. 1A shows the blade (B) wound on pulleys (P) during the lowering and cutting of the logs (L); FIG. 1B shows blade (B) which, during the lifting phase, cuts the logs (L) disposed again in the cutting position.
A drawback is due to the fact that the same point of the bevel, both the upper and the lower ones, comes into contact with the material of all the logs to be cut, before it is sharpened. The scheme of FIG. 1C shows that positions (P1, P2, . . . Pn), which are occupied by any point of the bevel, are cutting positions on various logs. In other words, as the blade (B) is horizontal, a same point of the bevel which is being used is obliged to pass through a plurality of logs (L) before exiting the cutting area. As a result, the cutting on the logs which are more downstream with respect to direction (D) of winding of blade (B) is carried out by means of a bevel which does not work in optimal conditions because it becomes less and less sharp while passing through a plurality of logs. Yet in other words, as the blade is horizontally oriented, the cutting of the logs which are downstream with respect to the direction (D) is carried out by means of a rather worn bevel. The quality of the cutting is consequently reduced.
A further inconvenience is due to the fact that the front portions of the logs (L), that is to say the portions (R) which constitute the commercial format rolls, are free. Then, the portions (R) of logs (L) tend to move during cutting on due to the thrust exerted on them by the blade (B) its lowering and lifting. This worsens the quality of the cutting, too.